Electromagnetic wave generator



Dec. 17, 1957 GoLDsTElN ETAL 2,817,045

ELECTROMAGNETIC WAVE GENERATOR 2 Sheets-Sheet 1 Filed Feb. 5, 1952 INVENTORS LADlsLAs GoLDs-rsm MURRAY A. LAMPERT BY .JOHN F. HEMD' A'TIORN Y l.. GoLDsTElN 1=:r AL 2,817,045

ELECTROMAGNETIC WAVE GENERATOR Dec. 17, 1957 2 Sheets-Sheet 2 Filed Feb. 5, 1952 INVENTORS LADISLAS GOLDSTEIN MURRAY A. LAMPERT BY JOHN F. @Ey

ATTORNEY United States Patent() ELECTROMAGNETIC WAVE GENERATOR Ladislas Goldstein, Urbana, Ill., Murray A. Lampert, New York, N. Y., and John F. Heney, Clifton, N. J., assignn ors to International Telephone and Telegraph Corporation, a corporation of Maryland Application February 5, 1952, Serial No. 270,034

Claims. (Cl. 315-39) This invention relates to electromagnetic wave generators and more particularly to means for generating electromagnetic waves having extremely short wavelengths.

At the present time there is a great need for millimeter wave generators. The term millimeter wave as used herein refers to electromagnetic wave energy having a Wavelength less than one centimeter. Redesigning centimeter wave generators to produce millimeter waves results in mechanical demands which are extremely diicult to meet, and even when these mechanical demands are met, the resulting generators are very fragile and capable of only very little power output.

One of the objects of this invention, therefore, is to produce an improved generator of ultra-high frequency electromagnetic waves without the aforementioned difficulties.

Another object of this invention is to provide a device for the conversion of direct currrent energy into radio frequency energy through the agency of the oscillations of electrons in a uniform unidirectional magnetic eld.

According to a feature of our invention, we provide a gaseous medium having free electrons Within a substantially uniform unidirectional magnetic lield. This gaseous medium having free electrons is an electron beam and/ or an electron gas created in an electric discharge. It is produced through the agency of an electric field whose lines of force are parallel with the lines of force of the aforementioned magnetic field. Appropriate waveguide structures are provided as output coupling means for the oscillating energy of the electrons.

A further feature of this invention is the addition of auxiliary electrodes to supply an additional electric field, transverse to the first mentioned electric lield and therefore also to the magnetic iield, to produce circular or spiral components of motion of the electrons in the gaseous medium.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent by reference to the following description taken in conjunction with the accompanying drawings, in which:

Fig. 1 is a diagrammatic perspective view partly in cross section of one form of millimeter wave generator in accordance with the principles of our invention;

Fig. 2 is a perspective view of an alternate embodiment of this invention having an oblique waveguide section; and

Figs. 3 and 4 are schematic illustrations of millimeter Wave generators having cylindrical resonant cavities in accordance with the principles of this invention.

Referring to Fig. l of the drawing, the millimeter wave generator shown therein includes a rectangular waveguide section 1 closed at one end 1a. A substantially uniform unidirectional magnetic eld is provided by a permanent magnet 2. In lieu of the permanent magnet 2 solenoids or electromagnets may be utilized to provide the magnetic eld across a section of waveguide 1. An electric discharge is provided parallel to the magnetic field by means of electrodes 3 and 4. The center of electrodes 3 and 4 may be spaced substantially a quarter wavelength or odd multiple thereof from the closed end 1a of waveposed of an appropriate dielectric such as glass.

2,817,045 Patented Dec. 17, 1957 guide 1. In some applications of this device where it is desired to match a load, the distance between the center of electrodes 3 and 4 and the closed end of waveguide section 1 may vary substantially from a quarter wavelength. The electrodes 3 and 4, properly insulated as indicated at 3a and 4a, may be part of the magnets pole faces 2a and 2b, or they may be part of the walls of waveguide 1 as shown. The space in the waveguide between magnetic pole faces 2a and 2b, hereafter called the interaction volume, may be sealed of by walls 5 and 6 which are com- The interaction volume will contain a suitable gas or mixture of gases at a suitable pressure or an electron beam in a high vacuum.

When the interaction volume contains a gas, such as helium, or a gas mixture, such as neon and argon, at an appropriate pressure, in the range of 10-3 to l0 millimeters Hg, a dense gas discharge plasma is created by an electric discharge between electrodes 3 and 4. The electrons in this plasma, in the absence of a magnetic eld, will tend to oscillate at an angular frequency op dependent upon the electron density No, the dielectric constant of free space e0, the electron e, and mass m according to the formula:

with all quantities measured in MKS units. The frequency The freis termed the electron gyroresonant frequency. When the electron gyroresonant frequency fH, as determined by the intensity of the magnetic lield, is substantially equal to the frequency fp of the plasma electron oscillations, the electron oscillations will be exceptionally strong, as a resonance phenomenon. This oscillating electron gas will radiate electromagnetic energy into waveguide 1b at substantially-the gyroresonant frequencies and harmonics thereof. Appropriate waveguide structures and filters can be added so that the output will be at the desired frequency, either a fundamental or a harmonic thereof.

To insure that the electrons traversing the waveguide section between electrodes 3 and 4 will not just follow a straight line path but will have some component of oscillating, angular motion such as is present if the electron path be spiral, auxiliary electrodes 7 and 8 may be added. The additional voltage between electrodes 7 and 3, creating an electric field perpendicular to the electron path between electrodes 3 and 4 and the magnetic lines of force, provides an additional source of energy to force each individual electron to follow a path having an oscillating, angular component of motion. If the additional voltage source, electrodes 7 and 8, is present, then there need not be a resonance of the plasma frequency fp and the electron gyrofrequency fH to obtain strong radiations of electromagnetic energy into the waveguide structure. For any electron density in the plasma, there will be strong radiation if the electron motion has a strong oscillating, angular component.

In the absence of a gas discharge plasma, an electron emissive cathode can be utilized to provide an electron beam in vacuum between electrodes 3 and 4. This electron beam will not have sufficient electron density to provide useful medium-like oscillations, and thus the additional electric field provided by auxiliary electrodes 7 and S is necessary to force each electron in the electron beam to oscillate at gyroresonant frequency fH. The oscillating electrons will radiate electromagneticV energy into the approximate waveguide structure at gyroresonant frequency and harmonics thereof, as heretofore explained.

Referring to Fig. 2, a millimeter wave generator similar to the device of Fig. l is shown comprising a rectangular waveguide 9 having a section 13 thereof at an oblique angle to the magnetic field lines of force from a magnet 10. Magnet 10 provides a substan-tially uniform unidirectional magnetic field across the interaction volume of the waveguide contained between the magnetic pole faces 10a and 10b. The interaction volume contains an electron gaseous medium as in the device of Fig. 1. Electrodes 11 and 12 are constructed parallel to magnetic pole faces 10a and 10b and at an acute angle to the longitudinal axis of section 13. The electric field between electrodes 11 and 12 is parallel with the magnetic field between pole faces 10a and 10b. The axis of the section 13 of waveguide containing the interaction volume forms an obliquer angle with lines of force of the magnetic field.

The energy from the electrons which is radiated into the waveguide of Fig. 1 might be termed an end-fire radiation, that is -the energy is radiated in substantially the same plane as the oscillating angular component of motion of the electrons. If the broadside radiation, that is radiation in a direction perpendicular to the angular component of motion of the electrons, is stronger than the end-fire radiation, then the waveguide section 13 of Fig. 2, being oblique to lthe magnetic lines of force, has a more favorable geometry for launching electromagnetic radiation into the transmission waveguide 9a because the angle between the axis of waveguide section 13 and the broadside direction is substantially less than 90 degrees.

Referring to Fig. 3 of the drawing, the ultra-high frequency generator therein shown includes a source of -electrons 14, a cylindrical cavity section 15 and an electromagnetic coil 16 disposed concentrically about the cavity section 15. The electromagnetic coil 16 is terminated on each side by end plates 17 and 1S which are composed of a material having a high permeability. End plates 17 and 18 confine the magnetic field to the cylindrical cavity 15. The cylindrical wall 15a comprises a non-magnetic conductor. The source of electron-s 14, external to the magnetic field, comprises a cathode 19 and an annular beam focusing electrode 20. A negative voltage is applied to cathode 19 by voltage source 21, and the focusing electrode 20 is biased negatively with respect to cathode 19.

The beam of electrons from electron source 14 traverses the cylindrical cavity 15 through the central aperture 22 in end plate 17 to the anode 23. Anode 23 is insulated from end plate 18 by dielectric ring 24, and a positive potential is applied to the anode from voltage source 25 through resistor 26. Auxiliary electrodes 27 and 2S provide an electric field transverse to .the electron beam from electron source 14.

In the absence of a magnetic field, any electron oscillations in -the cylindrical cavity take place in response to the radio-frequency vfields in the cavity and therefore have a frequency determined by the geometry of the cavity. The addition of the transverse electric field provided by electrodes 27 and 2S and the axial magnetic field provided by the magnet 16 produce electron motions with oscillating components at the electron gyroresonant frequency. At the resonance of the electron gyroresonant frequency and the radio frequency cavity frequency, the interaction of the electrons withI the radio frequency field of the cavity resonator will cause strongelectromagnetic waves to be produced which may be coupled to a load by means of a coaxial loop 29.

One variation of the device of Fig. 3 which will be equally effective to produce electromagnetic waves of extremely short wavelengths is shown in Fig. 4. The anode 30 of Fig. 4 is connected to a source of negative voltage 31 -through resistor 32, thus instead of functioning as a collector electrode as the anode 23 of device in Fig. 3, it now acts as a repeller electrode. By thus repelling the electron stream from cathode 19, an electron cloud is formed within the cylindrical cavity 15. The electrons within this electron cloud are caused to oscillate in a manner heretofore described. A probe 29 inserted into cylindrical cavity 15 couples the oscillatory energy from the electron cloud to a load.

While we have described above the principles of our invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of our invention as set forth in the objects thereof and in the accompanying claims.

We claim:

1. An electromagnetic wave generator comprising a hollow waveguide structure, an ionizable gaseous medium within said structure, electrode means to produce an electron flow along a. given path through said medium to ionize said medium and produce a dense gas discharge plasma whose electrons tend to oscilla-te at a given plasma frequency, means for producing a unidirectional magnetic field in said medium with its lines of force parallel with said given pathand of such a magnetic flux density as to cause the electrons of said plasma to oscillate at a gyroresonant frequency substantially equal to the frequency of the plasma oscillations, and output coupling means associated with said waveguide structure for coupling electromagnetic wave energy radiated by the electron oscillations from said medium.

2. An electromagnetic wave generator according to claim l, wherein said hollow waveguide structure includes a rectangular waveguide having its major transverse axis parallel to said lmagnetic lines of force.

3. An, electromagnetic wave generator according to claim 1, wherein said electrode means to produce anelectron flow includes electrodes positioned on opposite sides of said waveguide structure and means to insulate said electrodes from said waveguide.

4. An electromagnetic wave generator according to claim 1, wherein said means to produce a magnetic field includes a magnethaving pole faces disposed on opposite sides of said waveguide. Y

5. An electromagnetic wave generator according to claim 1, wherein the density of the electrons in the plasma and the magnetic liiux density are such as to substantially fulfill the equation rJ62N0 .mveo m where e=electron charge, B=magnetic flux density, m: electron mass, eo=dielectric constant of the plasma, and No=density of free electrons.

References Cited in the tile of this patent UNITED STATES PATENTS 2,521,760 Starr Sept. 12, 1950 2,558,664 Pease June 26, 1951 2,579,654 Derby Dec. 25, 1951 2,580,007 Dohler Dec. 25, 1951 2,591,350 Gorn Apr. 1, 1952 2,602,908 Lindery July 8, 1952 

